Polyiron(III)-Alkoxo Clusters: a Novel Trinuclear Complex and Its

Aug 1, 1995 - A T-Shaped μ 3 -Oxido Trinuclear Iron Cluster with High Easy-Plane Anisotropy: Structural and Magnetic Characterization. Pablo Alborés...
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Inorg. Chem. 1995, 34, 4660-4668

Polyiron(II1)-Alkoxo Clusters: A Novel Trinuclear Complex and Its Relevance to the Extended Lattices of Iron Oxides and Hydroxides A. Caneschi,? A. C o d a , ’ A. C. Fabretti,’ D. Gatteschi,**+and W. Malavasi’ Departments of Chemistry, University of Florence, Florence, Italy, and University of Modena, Modena, Italy Received January 13, 1995@

The synthesis, solid-state structure and magnetic characterization of the complex KFe3(OCH3)7(dbm)y4CH30H (I) (Hdbm = dibenzoylmethane) are reported. Complex I readily assembles in methanolic solutions of iron(II1) chloride and Hdbm in the presence of an excess of potassium methoxide; it crystallizes in the triclinic space group Pi with unit-cell parameters a = 13.044(1) A, b = 14.174(1) A, c = 18.657(2) A, a = 97.94(1)’, p = 104.13(l)’, y = 114.12(l)’, V = 2940.6(6) A3,and Z = 2 at 190 K. Refinement of 10076 reflections with 705 parameters yielded R = 0.046 and R , = 0.051. The solid-state molecular structure of I consists of a triangular array of iron(II1) ions connected by a triply bridging methoxide and three p2-methoxide ligands. The oxygen donors of a monodentate methoxide and of a chelating dbm ligand complete the coordination sphere of each metal ion. The resulting mononegative Fe3Cu3-OCH3)Cu2-OCH3)3(OCH3)3(dbm)3 moiety coordinates the K counterion through the oxygens of the p2-OCH3ligands. The oxygen atoms in the core of I are arranged in two essentially parallel layers and display a closest-packing which is found in iron oxides and hydroxides. The highspin iron(II1) ions are antiferromagnetically coupled with an S = ‘/* spin ground state. Assumption of a CzL point-group symmetry for the cluster leads to a satisfactory reproduction of the observed magnetic behavior with g = 2.0 and either J = 10.6(l), J’ = 15.3(2) cm-’ or J = 12.9(2), J’ = 9.7(1) cm-I, where the spin-only Heisenberg Hamiltonian is defined as H = &-,J,lS;S, and J’ is the unique coupling constant. The relative contributions of geometrical distortions and of antisymmetric exchange to the splitting of the 4-fold *E electronic ground state of an idealized system with trigonal symmetry have been estimated from X-band EPR spectra recorded on powdered samples

Introduction In recent years, the hydrolytic chemistry of iron(II1) has been actively investigated. Polyiron(II1) aggregates which exhibit some structural features typical of the extended lattices of iron(II1) oxides and hydroxides have been isolated from aqueous environments containing simple iron(II1) salts, a base, and suitably designed ligands which block the growth of the bulk oxides, limiting the size of the particle^.^,^ Some similarities are apparent between the structure of a-FeO(0H) (gothite)5 and the arrangement of the oxygen atoms from OH- and 0’- ligands observed in the complexes [Fel7(O)4(oH)l&eidi)8(H20)lrl3+ (1) and [Felg(O),j(OH)14(heidi)lo(H*O)12]+ (2).“,j Controlled To whom correspondence should be addressed. ’ University of Florence. University of Modena.

Abstract published in Aduance ACS Abstracts, August 1. 1995. ( I ) Schneider, W. Comments Inorg. Chem. 1984, 3. 205. Schneider, W. Chimia 1988. 42. 9. Hagen. K. S. Angew. Chern.. hit. Ed. Engl. 1992, 31. 1010. (2) Lippard, S. J. Angew. Chem., Int. Ed. Engl. 1988, 27. 344 and references therein. Lippard, S. J. Chem. Br. 1986. 22, 222. 13) Wieghardt. K.: Pohl, K.; Jibril, I.: Huttner. G. Angen. Chem., Int. Ed. Engl. 1984, 23. 77. Jameson. D. L.: Xie, C. L.; Hendrickson, D. N.; Potenza. J. A.: Schugar. H. J. J. Am. Chem. Soc. 1987. 109. 740. Harding. C. J.: Henderson. R. K.: Powell, A. K. AngeK,. Chem.. Inr. Ed. Engi. 1993. 32, 570. (4) Powell, A. K.; Heath. S . L.; Gatteschi, D.: Pardi, L.; Sessoli, R.: Spina. G.; Del Giallo. F.: Pieralli, F. J. Am. Chern. Soc. 1995, 1 17. 249 1, (5) Hyde. B. G.: Anderson. S. Inorganic Cnstal Structures: John Wiley & Sons: New>York. 1989; p 72.

161 Abbreviations used in the text: Hiheidi = N(CH?COOH)?(CH?CH>OH); Hdhrn = dibenzoylmethane (1.3-diphenyl- 1,3-propanedione); Hdpm = dipivaloylmethane (2.2,4.4-tetramethyl-3,5-heptanedione): HUC(~C = acetylacetone (2.4-pentanedione): tren = 2.2’.2”-triaminotriethylamine: H d h y = 3.5-di-tert-butylsemiquinone: Hdhcat = 3.5di-tert-but) Icatechol.

hydrolysis of iron salts represents indeed an excellent route to nanostructured materials and provides a means to study the boundary between molecular and extended systems.’-I0 Interestingly, it has been shown that hydrolytic-aggregation processes can be of use in the deposition of thin-layers of iron oxides from simple iron(II1) precursors by the sol-gel technique.’ The discovery that the catalytic sites of a number of nonheme iron proteins contain oxo- or hydroxo-bridged diiron unitsZ and the relevance of large polyiron(I1,III)-oxo aggregates to the ferritin core8.10has further encouraged this kind of studies. Iron(I1,III) polynuclear aggregates have been assembled in non-aqueous environments, taking advantage of the presence of water traces or of dioxygen to build up iron-oxo cores. Among the most recent examples are the dodecanuclear mixedvalent clusters Fel ?(O)~(CH~CO~)~(CH~O)IX(CH~OH)~.~~ (3)9and Fel2(0)*(CClHzC02)5,3Clo 7(CH30)18(CH30H)4 (4);’O their unusual molecular architecture is based on a closely packed array (7) Gatteschi. D. Adu. Mater. 1994, 6, 635. Gatteschi. D.; Pardi, L. Mol. Cysr. Liq. Cnlsr. 1993, 233. 217. Papaefthymiou. G. C. Phys. Reu. B 1992, 46. 10366. Delfs, C. D.: Gatteschi, D.; Pardi, L.: Sessoli. R.: Wieghardt. K.; Hanke, D. Inorg. Chem. 1993. 32. 3099. (8) Micklitz, W.; McKee. V.: Rardin, R. L.; Pence. L. E.; Papaefthymiou. G. C.: Bott, S . G.: Lippard, S. J. J. Am. Chem. Soc. 1994, 116. 8061. Micklitz, W.; Lippard, S. J. J. Am. Chem. Soc. 1989. 111. 6856. Gorun. S . M.; Papaefthymiou, G. C.; Frankel. R. B.; Lippard. S. J. J. Am. Chem. Soc. 1987. 109, 3337. Gorun. S. M.: Lippard. S. J. Nature 1986, 319, 666. (9) Taft, K. L.: Papaefthymiou, G. C.; Lippard, S. J. Inorg. Chern. 1994, 33. 1510. Taft, K. L.: Papaefthymiou. G. C.: Lippard. S . J. Science 1993, 259. 1302. (10) Caneschi. A,: Cornia. A,: Lippard. S. J.; Sessoli, R. Unpublished results. (11) Brinkler, C. J.: Scherer. G. W. Sol-Gel Science: The Physics and Chemistn of Sol-Gel Processing; Academic Press: New York, 1990. Guglielmi, M.: Principi. G. J. N h - C n s t . Sol. 1982. 48. 161.

0020- 1669/95/1334-4660$09.00/0 0 1995 American Chemical Society

Polyiron(II1)-Alkoxo Clusters Table 1. Crystal Data and Experimental Parameters for KFe3(OCH&(dbm)y4CH3OH chem. formula:*F ~ ~ K C S ~ H ~ O Ofw I ~= 1221.8 a = 13.044(1) space group:" Pi (No. 2) T = 190K b = 14.174(1) A c = 18.657(2) A 1 = 0.71069 A a = 97.94( I)" @calcd = 1.38 g cm-' p = 104.13(1)" p = 8.1 cm-I y = 114.12(1)0 Rb = 0.046 V = 2940.6(6) A3 RWb= 0.05 1 z=2 Hahn, T., Ed. International Tables for X-ray Crystallography, D. Reidel: Dordrecht, The Netherlands, 1983. R = x;llFol - lFciI/cIFol and R, = ~ w " 2 i l F o l- lFclI/Cw1'21FoIwhere w = 2.19/[a2(IFoI)f 3 x l0-"Fol?].

of oxygen atoms which simulates a fragment of a face-centered cubic lattice whose octahedral interstices are occupied by iron atoms. Two p6-oxo ligands represent fundamental structural elements in these peculiar systems. Our current efforts in this area involve basic alcoholic media and P-diketonates as limiting (and solubilizing) agents. P-Diketonates have complexing properties not dissimilar from those of certain biogenic ligands such as sugars, which display good limiting properties in aqueous solutions. The resulting iron(II1)-alkoxo cores are excellent structural models for the corresponding oxo-hydroxo systems. We herein report the synthesis, solid-state structure and magnetic behavior of a novel triiron(II1) cluster with a "voidedcubane" structure which readily assembles in strongly alkaline methanolic solutions of iron(II1) chloride and the ligand Hdbm.6

Experimental Section Synthesis. All operations were performed with strict exclusion of moisture, unless otherwise stated. Methanol was distilled over Mg(OCH& shortly before use. Hdbm (Aldrich, 98%) and resublimed FeC13 (Carlo Erba, 99%) were used without purification. Potassium methoxide was used as a 2 5 8 solution in methanol (Flukaj. To I O mmol of iron(II1) trichloride dissolved in 30 mL of anhydrous methanol was added a solution containing 10 mmol of Hdbm and 40 mmol of potassium methylate in 120 mL of anhydrous methanol dropwise with stirring at 298 K. After the addition was completed, the mixture was stirred for I h; the yellow precipitate was then filtered off, and the resulting bright-yellow solution was allowed to stand at room temperature. Yellow rod-like crystals formed overnight; upon further standing, they were gradually replaced by yellow diamondshaped crystals which were collected by filtration, rapidly washed with methanol, and dried under vacuum (approximate yield 70%). Both types of crystals gave satisfactory analyses for Fe3KC56H7001,. IR spectral data (KBr pellets, cm-I): 3422 (br), 3061 (s), 2920 (s), 2816 (s), 1594 (s), 1549 (s), 1522 (s), 1479 (s), 1452 (s), 1376 (s), 1318 (s), 1226 (m), 1182 (w), 1072 (s), 1024 (s), 941 (w), 785 (m), 753 (mj, 721 (s), 688 (s), 620 (s), 497 (s), 461 (s). UV reflectance spectral data (nm): 375 (s), 450 (sh). X-ray Data Collection. A yellow, diamond-shaped crystal (0.3 x 0.5 x 0.4 mm) was removed from the mother liquid, mounted on the top of a thiny glass fiber with silicon grease under a cold dinitrogen stream, and rapidly transferred to an Enraf-Nonius CAD4 diffractometer equipped with low-temperature facility and Mo-Ka graphite-monochromated radiation. Unit cell parameters were determined by leastsquares fitting to the setting angles of 24 intense low-angle reflections (9.7 < 0 < 15.0"). The crystal quality was judged satisfactory on the basis of the average half-height width of sample reflections ( ~ 1 1 2 ) = 0.29'. Crystal data and relevant experimental parameters are reported in Table 1. Laue group was confirmed by running the program TRACER II.'? All data were corrected for Lorentz and polarization effects. No absorption correction was applied to collected intensities. (12) Lawton, S. L. Tracer 11: A Fortran Lattice Tranrformation-Cell Reduction Program: Mobil Oil Corp.: Paulsboro, NJ, 1967.

Inorganic Chemistry, Vol. 34, No. 18, 1995 4661 Structure Solution and Refinement. The structure was solved by direct methods with the program SIR92,13 which gave the positions of all non-hydrogen atoms. Refinement was carried out by using the SHELX76 program package.I4 [lo076 reflections with F: > 4a(Fo?)]. Anisotropic thermal parameters were refined for all non-hydrogen atoms. Hydrogen atoms H(014) and H(015) were located in AF maps, whereas all the remaining hydrogen atoms were treated as fixed contributors in calculated positions and allowed to ride on the attached carbon atoms. The hydrogen atom attached to O(16) was not located. Isotropic thermal parameters B(H) = 1.2 B,,(X), where X is the parent oxygen or carbon atom, were used for all hydrogen atoms. Final atom positional parameters for non-hydrogen atoms are reported in Table 2. Relevant bond distances and angles are summarized in Table 3. Selected non-bonded distances and angles for the KFe3 tetrahedron can be found in Table 4. Instrumentation and Physical Measurements. The magnetic susceptibility of a 20.2 mg microcrystalline sample was measured by using a Mttronique MS03 SQUID Magnetometer in the temperature range 2.2-270 K in an applied field of 1 T. The contribution of the sample holder was determined separately in the same temperature range and field. Diamagnetic corrections were estimated from Pascal's constants. EPR spectra of powdered samples were recorded on a Varian E9 spectrometer operating at X-band frequency, equipped with an Oxford Instruments ESR9 liquid-helium continuous-flow cryostat, in the temperature range 4.2-298 K. Least-Squares Calculations. All calculations were performed on a CONVEX 220 computer using the E04FCF-NAG Fortran Library minimization routine.

Results and Discussion Synthesis. The possible successful use of P-diketonates as limiting agents for the study of solvolytical aggregation processes involving iron(I1,III) has been suggested by the previous isolation of low-nuclearity polyiron species containing alkoxide bridges and chelating P-diketonate ligands. Twenty years ago Gray and co-workers r e p ~ r t e d 'the ~ synthesis, spectroscopic and magnetic characterization of dialkoxo-bridged iron(II1) dimeric complexes formulated as Fez(OR)@)d, where HL = Hacac or Hdpm6 and R = CH3, C2H5, or i-C3H7. These complexes were found to assemble readily under aerobic conditions by reaction of simple iron(I1,III) salts with the ligand HL and a base using the appropriate alcohol as a solvent. More recently the tetranuclear cubane complexes Fett(OCH3)(CH30H)(6) were isolated (dbm)]4( 5 ) and [FeIt(0CH3)(CH30H)(dpm)]4 by Lippard et al. l 6 as products of the reaction of iron(I1) chloride with 1 equiv of P-diketone and 2 equiv of lithium methoxide in methanol under strictly anaerobic conditions. In the course of a systematic study on the limiting properties of P-diketonates in alcoholic media, we observed that reaction of iron(II1) chloride with 1 equiv of Hdbm and 4 equiv of potassium methoxide in anhydrous methanol leads to the precipitation of a mixture of a bright yellow solid and potassium chloride. By slow evaporation of the filtered yellow solution, yellow needlesI7 are obtained overnight in low yield. Upon further concentration, large diamond-shaped yellow crystals (13) Altomare, A,; Cascarano, G.: Giacovazzo, C.: Guagliardi. A. J. Appl. C y t a l l o g r . 1994, 27, 1045. (14) Sheldrick. G. M. SHELX76, Program for C q m l Structure Determination; University of Cambridge: Cambridge, England, 1976. (15) Wu. C.-H. S . ; Rossman, G. R.: Gray, H. B.: Hammond. G. S . ; Schugar, H. J. Inorg. Chem. 1972, 11, 990. (16) Taft. K. L.; Caneschi, A,; Pence, L. E.: Delfs, C. D.; Papaefthymiou, G. C.: Lippard S . J. J. Am .Chem. Soc. 1993, 115. 11753.

4662 Inorganic Chemistly, Vol. 34, No. 18, 1995 Table 2. Final Atom Positional Parameters and B(eq) Values atom

rla -0.13992(3) -0.28384(3) -0.06190(3) -0.34962(6) -0.1044( 2 j -0.3044(2) -0.2336(2) -0.0995(2) 0.0302(2) -0.1582(2) -0.3176(2) -0.2265(2) -0.0 17 I(2) 0.1 134(2j -0.1956(2) -0.4446(2) -0.0535(2) -0.2228(2) -0.334.5(3) -0.5499(3) -0.0239(3) -0.40 18(3) -0 2840(3) -0.0380(3) 0.0766(3) 0.0235(3) -0.0898(3) -0.3238(2) -0.2757(3) -0.23 17(2) 0.0839(3) 0.1927(3) 0.2014(3) -0.1827(4) -0.5095(3) 0.03 lO(4) -0.2571(4) -0.35 12(4) -0 6456(4)

vlb 0.01051(3) -0.23942(3) -0.16287(3) -0.17263(6) -0.1 165(1) -0.1 164(1) -0.2673( 1) -0.0429( I ) 0.1305(2) 0.050l(2) -0.2035(2) -0.3342(2) -0.2755(2) -0.0604( 2) 0.0973(2) -0.3448(2) -0.1975(2) -0.1991(2) 0.0166(2) -0.3314(3) -0.0952(2) -0.0975(2) -0.3714(2) 0.0301 (2) 0.2046(2) 0.2058(2) 0.1283(2) -0.2508(2) -0.3209(2) -0.3604(2) -0.2747( 2) -0.1828(2) -0.0815(2) 0.1966(3) -0.4291(3) -0.2177(4) -0.2901(3) 0.0896(4) -0.38 19(4)

dC 0.72407(2) 0.72007(2) 0.65542(2) 0.53 10 l(4) 0.7546( 1'1 0.6906( 1) 0.6293( I ) 0.6360( I ) 0.7738( I ) 0.826 1( 1) 0.8183( I ) 0.7710( 1) 0.6895( I ) 0.7069( I ) 0.6764( I ) 0.6697( I ) 0.5569( I ) 0.4446( I ) 0.53 l3(2) 0.41 8 l(2) 0.8298(2) 0.6979(2) 0.5785(2) 0.597 l(2) 0.8363(2) 0.8929(2) 0.8855(2) 0.87 16(2) 0.8850(2) 0.8332(2) 0.7041(2) 0.7232(2) 0.7224(2) 0.7 140(2) 0.6967(3) 0.5332(2) 0.3868(2) 0.4929(3) 0.3489(3)

Caneschi et al.

(A2)for KFej(OCH?)7(dbm)?*4CH30Ht' B(eq)' 1.74( 1j 1.70(I ) I .76(2) 3.02(3) 1.75(7) 1.81(7) 1.85(7) 1.93(7) 2.31(7) 2.42(8) 2.20(8) 2.07(7) 2.27(8) 2.25(8) 2.65(8) 2.61(8) 2.55(8) 3.5(1) 5.9(1) 6.6( I ) 2.3(1) 2.6(1) 2.8( I ) 2.6( 1j 2.3(1) 231) 2.3(1) 1.9(1) 2.2( 1 ) 2.0(1) 2.2( 1 j 2.3( 1) 2.1(1) 4.2(2) 4.7(2) 4.1(2) 4..5(2) 5.6(2) 5.6(2)

atom

\la 0.1960(3) 0.2692(3) 0.3819(3) 0.4227(3) 0.3493(3) 0.2376(3j -0.1371(3) -0.0923( 3) -0.1378(3) -0.2229(4) -0.27 13(5) -0.2289(4) -0.3966(3) -0.3785(3) -0.4505(4) -0.5434(4) -0.5648(4) -0.4895(3) -0.1894(3) -0.2 197(3) -0.1743(3) -0.1004(3) -0.0716(3) -0.1 164(3) 0.0820(3) -0.0177(3) -0.0227(3 j 0.0721f3) 0.1708(3) 0.1766(3) 0.32 1 l(3) 0.4234(3) 0.5332(3) 0.542 l ( 3 ) 0.441 3(3) 0.330 1(3j -0.5930(3) -0.6993(4)

+

vlb 0.2947(2) 0.2781(3) 0.3597(3) 0.4.599(3) 0.4769(3) 0.3962(2) 0.1330(2) 0.2253(3) 0.2220(3 j 0.1295(3) 0.0382(3) 0.0404(3) -0.2344(2) -0.2409( 3) -0.2268(3) -0.2 l06(4) -0.2075(5) -0.2 172(4) -0.44 13(2) -0.501 l(2) -0.5727(3) -0.5857(3) -0.5290(3) -0.4572(2) -0.3826(2) -0.4714(3) -0.5726(3) -0.5 849(3) -0.4982(3) -0.3968(3) 0.0093(2 j 0.0207(3) 0.105 l(3) 0.1785(3) 0. I68 l(3) 0.0843(3) -0.3454(3) -0.3595(5)

"Numbers in parentheses are errors in the last significant digit. "B(eq) = (8x213)[Ul~(aa*)' U??(bb*)?+ Ui?(cc*)' 2U~gia*cc*cos/3 + 2U?;bb*cc*cos a].

grow in 1-2 days and gradually replace the needle-like crystals. The two crystalline phases gave very similar elemental analyses and were indeed shown by single-crystal X-ray techniques to be different crystalline forms of the novel triiron(II1) cluster KFe3(OCH3)7(dbm)3-4CH3OH (7).Surprisingly, use of lithium or sodium methoxide instead of potassium methoxide did not lead to the expected lithium- or sodium-containing compounds, respectively. On the basis of the available solid-state structure, we suggest that the potassium ion may exhert a specific template effect in the synthesis of 7 by virtue of its large ionic radius which allows a particularly favorable accomodation on the hollow side of the cluster. Modifications of the above decribed synthetic procedure allowed the isolation of a surprising variety of methoxo-bridged polynuclears. In addition to the well-known dimeric species Fez(OCH3)z(dbm)4 (8) a hexanuclear18 and a d e c a n ~ c l e a r ' ~ complex were synthesized and structurally characterized. The simple coordination requirements of P-diketonates, which have (17) FeiKC~,H700~7. fw = 1221.8;triclinic, Pi (No. 2): a = 12.760(4). h = 20.854(4). c = 24.452(4) A; a = 112.56(1).'I/ = 100.30(2). y = 92.42(2)": V = 5868(2) A'. Z = 4; dcaicd= 1.38 g cm-?: 14501 reflections collected at T = 220 K with Mo K a radiation; for 8164 reflections with F , > 4a(F,) and 695 parameters, R = 0.068. R,, = 0.07 1. (18) Caneschi, A.: Comia, A.: Lippard, S. J. Ange". Chrm., Irzr. Ed. Engl. 1995. 34. 467. (19) Caneschi. A.: Cornia, A.; Fabretti. A. C.; Gatteschi. D. Unpublished results.

:IC

0.8488(2) 0.81 1 l(2) 0.8235(2) 0.8727(2) 0.9095(2) 0.8980(2) 0.9503(2) 1.0092(2) 1.0693(2) 1.0723(2) 1.0128(3) 0.9517(2) 0.9186(2) 0.9933(2) 1.0330(2) 0.9979(2) 0.9225(3) 0.8837(2) 0.8491(2) 0.9010(2) 0.9 152(2) 0.8777(2) 0.8255(2) 0.8107(2) 0.6955(2) 0.6439(2) 0.6330(2) 0.6749(2) 0.7263(2) 0.7371(2) 0.7374(2) 0.7905(2) 0.8021 (2) 0 7591(2) 0.7050(2) 0.6942(2) 0.5477(2) 0.5623(3)

B(es)' 231) 3.1(1) 3.9(1) 4.3(2) 4.1(1) 3.2(1) 2.4(1) 3.1(1) 3.8(2) 4.2(2) 6.2(2) 5.0(2) 2.2( 1) 3.3( 1) 3.8(2) 4.8(2) 7.0(3) 5.0(2) 2.2(1) 3.0( 1) 3.6( 1) 3.5(1) 3.4(1) 2.7( I ) 2.3(1) 3.0( 1) 3.8(1) 3.9(2) 3.6(2) 3.0(1) 2.5( 1 ) 2.9( 1 ) 3.4( 1 ) 3.9( I ) 4.2(2) 3.3(1) 6.8(2) 7.2(3)

- 2U1?aa*bb*cosy

-

been found to act almost invariably as chelating ligands, may play an important role since they turn out to be compatible with a large number of structural types. Furthermore, the molecular structure and coordination geometry of P-diketonates [RC(O)CHC(O)R] are such that the peripheral hydrophobic shell of the cluster originates mainly from the R substituents. The nature of the R groups (polarity, steric hindrance, etc.) is thus expected to have a great influence on the displayed limiting properties. dbm (R = ChHs) apparently represents a particularly favorable choice in conjunction with polar solvents such as methanol or methanolkhloroform mixtures. Crystal Structure. Figure 1 shows an ORTEP view of compound 7,as resulting from single-crystal X-ray measurements at 190 K. The carbon atoms of the phenyl rings of the dbm ligands and all hydrogen atoms except for HO( 14), HO( 15), and HOs have been omitted for clarity. An alternative view of the cluster along its virtual Ci axis is presented in Figure 2 . The iron atoms lie at the vertices of an equilateral triangle, the iron-iron separations being equal within experimental error (see Table 4), and display distorted octahedral coordination geometries with oxygen donors only. Peculiar to complex 7 is the triply bridging methoxide ligand O( 1)-C( l), which represents a fundamental structural element. Each side of the triangle is further bridged by a p2-OCH3 ligand. A chelating dbm and a monodentate OCHi ligand per iron atom complete the metal coordination environments. The organic moieties stretch out of the cluster, acting as blocks to its further growth. The

Polyiron(II1)-Alkoxo Clusters Table 3. Selected Bond Lengths

Inorganic Chemistry, Vol. 34, No. 18, 1995 4663

(A) and Angles (deg) for

Table 4. Selected Nonbonded Distances KFe3(OCH&(dbm)3CH30Ho

KF~~(OCH~),(~~~)I.~CHIOH~ O( 1)-Fe( 1) 0(4)-Fe( 1) 0(6)-Fe( 1) O( 1)-Fe( 2) 0(3)-Fe(2) 0(8)-Fe(2) O( 1)-Fe(3) 0(4)-Fe(3) O( 10)-Fe(3) C(1)-0(1) C(3)-0(3) C(5)-0(5) C(8)-0(7) C( 11)-0(9) C(14)-O(11) C( 16)-O( 13) C( 18)-O( 15) cs-os K-O(3) K-O(14) K-O(16) HOS-Os HO(15)-O(15) O(2)-Fe( 1)-O( 1) O(5)-Fe( 1)-O( 1) O(11)-Fe(1)-O(1) 0(5)-Fe( 1)-O(2) O( 1 1)-Fe( 1)-O(2) O(6)-Fe( 1)-0(4) O(6)-Fe( 1)-O(5) O( 1 1)-Fe( 1)-O(6) 0(3)-Fe(2)-0( 1) O(8)-Fe(2)-0( 1) 0(3)-Fe(2)-0(2) 0(8)-Fe(2)-0(2) O( 12)-Fe(2)-0(2) O( 8)-Fe(2)-O(7) O( 12)-Fe(2)-0(7) 0(3)-Fe(3)-0( 1) 0(9)-Fe(3)-0( 1) O( 13)-Fe(3)-0( 1) 0(9)-Fe(3)-0(3) O( 13)-Fe(3)-0(3) O( 10)-Fe(3)-0(4) O( lO)-Fe(3)-0(9) O( 13)-Fe(3)-0( 10) Fe( 1)-O( 1)-Fe(3) C( 1)-O(1)-Fe(1) C( 1)-O( 1)-Fe(3) C(2)-O( 2)-Fe( 1) Fe(3)-0(3)-Fe(2) C(3)-0(3)-Fe(3) C(4)-0(4)-Fe( 1) C(5)-0(5)-Fe( 1) C(8)-0(7)-Fe(2) C( 1 1)-0(9)-Fe(3) C(14)-0(1l)-Fe(l) C(16)-O( 13)-Fe(3) O( 15)-K-0(16) O( 1.5-K-0(3) O( 1 5 ) - K - 0 ~ O( 14) -K- O(2) O( 14)-K-O(4) O( 16)-K-0(2) O( 16)-K-0(4) O(2) -K- O(3) 012)-K-0~ Oi3j-K-Os C( 17)-0(14)-H0(14) Cs-OS-HOS

2.148(2) 1.989(2) 2.008(2) 2.133(2) 2.005(2) 2.023(2) 2.15 l(2) 2.007(2) 2.022(2) 1.437(3) 1.423(3) 1.275(3) 1.275(4) 1.273(4) 1.406(5) 1.398(6) 1.394(7) 1.418(8) 2.905(2) 2.668(3) 2.744(2) 0.83(5) 0.74(4)

O(2)-Fe( 1) O(5)-Fe( 1) O( 1 1)-Fe( 1) 0(2)-Fe(2) 0(7)-Fe(2) O( 12)-Fe(2) 0(3)-Fe(3) O(9) -Fe( 3) O( 13)-Fe(3) C(2)-0(2) C(4)-0(4) C(7)-0(6) C( 10)-0(8) C( 13)-O(10) C( 15)-O(12) C( 17)-O(14) C( 19)-O( 16) K-O(2) K-O(4) K-O(15) K-Os HO( 14)-0( 14)

76.64(8) 0(4)-Fe( 1)-O( 1) 96.74(8) 0(6)-Fe( 1)-O( 1) 166.84(7) O(4)-Fe( 1)-0(2) 170.6(1) O(6)-Fe( 1)-0(2) 93.7( 1) 0(5)-Fe( 1)-0(4) 166.7(1) O(ll)-Fe(1)-0(4) 85.U 1) O( 11)-Fe( 1)-O(5) 97.8( 1) 0(2)-Fe(2)-0( 1) 76.86(7) 0(7)-Fe(2)-0( 1) 89.95(8) O( 12)-Fe(2)-0( 1) 91.3(1) 0(7)-Fe(2)-0(2) 165.5(1) 0(7)-Fe(2)-0(3) 96.4( 1) 0(8)-Fe(2)-0(3) 84.8(1) 0(12)-Fe(2)-0(3) 91.7(1) 0(12)-Fe(2)-0(8) 76.53(7) 0(4)-Fe(3)-0( 1) 97.13(9) 0(10)-Fe(3)-0(1) 166.8(1) 0(4)-Fe(3)-0(3) 88.9(1) O( 10)-Fe(3)-0(3) 96.2( 1) 0(9)-Fe(3)-0(4) 91.9( 1) O( 13)-Fe(3)-0(4) 84.9(1) 0(13)-Fe(3)-0(9) 96.0(1) Fe(2)-O(l)-Fe(l) 98.00(8) Fe(2)-0( 1)-Fe(3) 118.6(2) C(l)-O(l)-Fe(2) 119.2(2) Fe(2)-0(2)-Fe( 1) 118.7(2) C(2)-0(2)-Fe(2) 108.0(1) C(3)-0(3)-Fe(2) 121.3(2) Fe(3)-0(4)-Fe(l) 120.1(2) C(4)-0(4)-Fe(3) 129.7(2) C(7)-0(6)-Fe( 1) 129.7(2) C( 10)-0(8)-Fe(2) 127.9(2) C(13)-0(10)-Fe(3) 12.5.3(2) C( 15)-0(12)-Fe(2) 130.8(2) 0(15)-K-0(14) 111.6(1) 0(15)-K-0(2) 137.82(7) 0(15)-K-0(4) 115.5(1) 0(14)-K-0(16) 133.84(7) 0(14)-K-0(3) 76.82(6) O( 1 4 ) - K - 0 ~ 128.84(9) O( 16)-K-0(3) 161.9( 1) O( 1 6 ) - K - 0 ~ 59.97(7) 0(2)-K-0(4) 77.6016) 013)-K-014), , 85.05i9j o i 4 j - ~ - o s lOl(3) C(18)-0( 15)-H0(15) 109(4)

2.028(2) 2.038(2) 1.884(3) 1.998(2) 2.033(2) 1.897(2) 2.001 (2) 2.039(3) 1.877(2) 1.433(5) 1.434(4) 1.274(3) 1.275(4) 1.278(4) 1.395(5) 1.391(5) 1.40l(5) 2.82 l(2) 2.974(2) 2.607(4) 3.242(4) 0.73(4) 76.93(9) 90.93(9) 93.9(1) 88.4(1) 91.0( 1) 95. I( 1) 93.8(1) 77.63(8) 97.98(7) 168.59(9) 89.6(1) 174.4(1) 93 .O( 1) 93.7(1) 97.0( 1) 76.49(8) 92.40(8) 92.8( 1) 166.6(1) 172.8(1) 93.1( 1) 93.7(1) 98.40(9) 98.39(6) 119.8(2) 107.2(1) 12 1 3 2 ) 122.4(2) 108.6(1) 12 1.1(2) 13 1 3 2 ) 131.1(2) 128.7(2) 126.1(2) 105.0(1) 87.58(8) 82.33(8) 88.04(9) 83.77(8) 130.08(9) 109.87(9) 5 1.239) 60.80(6) 59.1815) 134.39i7j 119(5)

Numbers in parentheses are errors in the last significant digit.

[Fe3(OCH3)7(dbm)3]- core described so far approaches the C3a point symmetry if the nonplanarity of the dbm moieties is

Fe( 1)-Fe(2) Fe(2) -Fe(3) Fe(2)-K Fe(2)-Fe( 1)-Fe(3) Fe(2)-Fe(3)-Fe( 1) Fe(3)-Fe( 1)-K Fe(3)-Fe(2)-K Fe(l)-Fe(3)-K Fe( l)-K-Fe(3)

3.241( 1) 3.242( 1) 3.766( 1)

(A) and Angles (deg) for

Fe( 1)-Fe(3) Fe( 1)-K Fe(3)-K

59.99( 1) 59.95( 1) 66.20(2) 65.91(2) 63.13(2) 50.67(1)

Fe( 1)-Fe(2)-Fe(3) Fe(2)-Fe( 1)-K Fe( 1)-Fe(2)-K Fe(2)-Fe(3)-K Fe(l)-K-Fe(2) Fe(2)-K-Fe(3)

3.244( 1) 3.74 1(1) 3.837( 1) 60.06( 1) 64.82(2) 64.03( 1) 63.62(2) 5 1 . 1 3 1) 50.47(1)

fl Numbers in parentheses are estimated standard deviations in the last significant digit.

C(12I

Figure 1. ORTEP view of compound 7. The carbon atoms of the phenyl rings of the dbm ligands and all hydrogen atoms except for HO( 14), HO( 15) and HOs have been omitted for clarity. Thermal ellipsoids enclose 20% probability.

neglected. However, significant distortions from the above idealized symmetry show up on careful inspection of the metrical parameters reported in Table 3 and sketched in structure I. Two of the Fe-O( 1) bond distances are essentially equal to

I

each other [O(l)-Fe(1) = 2.148(2) A, 0(1)-Fe(3) = 2.151(2) A] whereas the third one is significantly shorter [0(1)-Fe(2) = 2.133(2) A]. Some distortions affect the pz-OCH3 bridges as well. While O(3) bridges in a symmetrical fashion, the 0(3)-Fe(3) and 0(3)-Fe(2) distances being equal within experimental error, O(2) and, to a lower extent, O(4) have

Caneschi et al.

4664 Inorganic Chemistry, Vol. 34, No. 18, 1995 C(35)

Figure 2. ORTEP plot of the FedOCH3)ddbm)j- moiety of compoud 7 as viewed along its virtual Cj axis. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids enclose 20% probability.

significantly different bond distances with the iron atoms of each appropriate pair. Small differences are also found in the bond distances of each iron center with the oxygen from the corresponding terminal methoxide ligand. Namely, O( 12)Fe(2) [ 1.897(2) A] is longer than the remaining two separations, which average 1.88 A. All the above observations can be satisfactorily rationalized by looking at the arrangement of the potassium ion and of the methanol molecules with respect to the [Fe3(OCH3)7(dbm)j]core. The iron centers and the potassium ion in 7 are found at the vertices of a distorted tetrahedron whose metrical parameters are summarized in Table 4. The potassium ion occupies the molecular hemicavity which results from the arrangement of the iron-oxygen framework of the trinuclear core and is coordinated by the three oxygen donors of the ,u?-methoxides. The K-O(2) separation [2.821(2) A] is significantly shorter than K-O(3) [2.905(2) A] and K-O(4) [2.974(2) A], which partially explains the lengthening of the 0(2)-Fe(l) distance. The methanol molecules are asymmetrically arranged. Two of them coordinate the potassium ion and are hydrogen-bonded to O( 11) and O( 13) of the monodentate methoxides with essentially equal 0 .O separations. The geometrical parameters of hydrogenbond interactions are given in Table 5. A third solvent molecule is coordinated to the potassium ion only. The last methanol molecule is loosely bound to the latter, while having a strong hydrogen bond with 0(12), a situation which may be reflected in the long 0(12)-Fe(2) separation. The shortening of the 0(1)-Fe(2) distance then follows as a trans effect. Though the hydrogen atom attached to O(16) could not be located in AF maps, the O(16)-Os separation [2.628(5) A] suggests that a hydrogen bond may be present between O(16) and Os as well. The triiron(II1)-methoxide core of complex 7 is an example of a 6-metallacrown-3 structure type in which the six-membered ring of iron(II1) and oxygen atoms Fe( 1)-0(2)-Fe(2)-0(3)-

Table 5. Hydrogen-Bond Distances

(A) and Angles

(deg) for

KFel(OCH3)7(dbm)j*4CH3OH HO( 14). Q(13 O( l4)-HO( 14). * O(13) HO(15)..*0( 1 1) 0(15)-HO(15). ' O(1 1) HOs. * SO(12) OS-HOS O(12) O( 16). - 0 s

1.91(4) 17 l(5) 1.92(4)

O( 14)- Q(13)

2.638(3)

0(15).*.0(11)

2.642(3)

Os**.O(12)

2.599(4)

167(6) 1.78(5)

170(5) 2.628(5)

Numbers in parentheses are estimated standard deviations in the last significant digit. I'

Fe(3)-O(4) binds the potassium ion on one side. Interestingly, the hexairon(II1)-methoxide cluster [MFes(OCH,),l(dbm),]. 12CH3OHCHC13 ( M = Na, Li) (9) displaying the related 12metallacrown-6 structure type and harboring the sodium or lithium ion in the center has been recently i s ~ l a t e d . ' ~ ~ ' ~ Examples of the 9-metallacrown-3?" and 12-metallacrown-4" structure types are also available. Compound 7 is structurally related to a number of polyiron(II1)-oxo aggregates studied in recent years. Complexes 3-6 all contain triangular arrays of iron atoms with p3-OCH3 ligands. Noticeably, the [Fe3(u3-OCH3)@2-OCH3)3(OCH3)3(dbm)3]- core observed in 7 shows striking similarities with the Fe3@3-OCH3)4(CH30H)?(L)jfragments which can be individuated in 5 and 6 (L = dbm, dpm, respectively). Indeed, the two latter compounds can be formally constructed from 7 by replacing the K(CH3OHh unit with a Fe(CH3OH)(L) unit and transforming the , u ~ - O C Hand ~ monodentate methoxides into pj-OCH3 and monodentate methanol molecules, respectively. (20) Lah. M. S . : Kirk, M. L.: Hatfield. W.: Pecoraro. V. L. J. Clzrrn. Soc.. Chrm. Commrrri. 1989, 1606. (21) Lah. M. S . : Pecoraro. V. L. J . Am. Chrm. Sot,. 1989. 1 / I . 7258.

Polyiron(II1)-Alkoxo Clusters Similar Fe3@3-OR)2@2-OR)2 fragments are also present in the complex Fe4(dbsq)4(dbcat)4(10).6,22 Several structural analogies with the three hexanuclear p.5oxo bridged clusters known so are also apparent. Their Fe6(u6-O) core can be imagined to derive from the fusion of two Fe3@3-OCH3) units, with suppression of the methyl groups and transformation of the methoxide oxygen atom into a p.5oxo ligand. Interestingly, the topology of the K atom in 7 is strictly similar to that of the Na counterions in Na2Fe6@6O)(OCH3)18*6CH30H(11),23 each alkali-metal ion being coordinated, in this second case, by three oxygen donors from p2OCH3 ligands in correspondence of a triangular face of the metal octahedron. In the hexairon(II1) cluster Fes(O)r(OCH3)12(tren);?(CF~S03)2*2CH30H (12),6,26 Fe3@4-0)(OCH3)3 and Fe3@4-0)2(OCH3)2 moieties can be easily recognized which bear structural resemblance to the core of 7. A comparison with other structurally characterized triiron(1II) complexes is also instructive. Most of them exhibit the wellknown symmetric p3-oxo-centered structure typical of "basic iron carboxylate^".'^ The central p3-oxide ligand is coplanar to the three iron(II1) ions which are further bridged by carboxylate or sulfate ligands.28 Asymmetric p3-0xo-centered~~ and linear30 trinuclear iron(II1) complexes have also been reported. In 7 the structure is more similar to that postulated for the active sites of [3Fe4S] protein^.^' The possible formation of heterometal clusters [ M F ~ ~ S J ] +[M . ~ += Co(II), Zn(II)] in the active sites of Desuljouibrio gigas Fd I1 has been demonstrated. Structurally, they are similar to the [KFe3@3-OCH3)@2-0CH3)3] core of compound 7. The conversion of the cubane mixed-valent (13) into [Fe3S4In (14) and [Fe3S41f (15) formally proceeds by removal of an iron(I1) ion and a subsequent one-electron oxidation step, respectively. However, as shown by cyclic voltammetry experiments of both 1,2-dichloroethane and acetoBoone, S. R.; Purser, G. H.; Chang, H.-R.; Lowery, M. D.; Hendrickson, D. N.; Pierpont, C. G. J. Am. Chem. Soc. 1989. I l l , 2292. Hegetschweiler, K.: Schmalle, H. W.; Streit, H. M.; Schneider, W. Inorg. Chem. 1990, 29, 3625. Hegetschweiler, K.; Schmalle, H. W.; Streit, H. M.; Gramlich, V.; Hund. H. U.; Emi, I. Inorg. Chem. 1992, 31, 1299. Comia, A.; Gatteschi, D.; Hausherr, L.; Hegetschweiler, K. Unpublished results. Nair. V. S.; Hagen, K. S. Inorg. Chem. 1992, S I , 4048. Scheurer-Kestner, M. Ann. Chim. Phys., Ser. 3. 1861, 63, 422. Weinland, R. F.; Hohn, A. 2. Anorg. Chem. 1926, 152, 1 . Welo, L. A. Philos. Mag. S7 1928, 6,481. Earnshaw, A,; Figgis, B. N.; Lewis, J. J. Chem. Soc. A 1966, 1656. Holt, E. M.; Holt, S. L.; Tucker, W. F.; Asplund. R. 0.;Watson, K. J. J. Am. Chem. Soc. 1974. 96, 2621. Heald, S . M.; Stearn, E. A,: Bunker, B.; Holt, E. M.; Holt, S. L. J. Am. Chem. Soc. 1979, 101, 67. Holt, E. M.; Holt, S. L.: Alcock, N. W. C y s t . Srrucf. Commun. 1982, 11, 505. Dziobkowski, C. T.; Wrobleski. J. T.; Brown, D. B. Inorg. Chem. 1981,20,671.Uemura, S.; Spencer, A.: Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973, 2565. Catterick, J.; Thomton, P.; Fitzsimmons, B. W. J. Chem. Soc., Dalton Trans. 1977, 1420. Catterick, J.; Thomton, P. Adv. Inorg. Chem. Radiochem. 1977, 20, 291. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1980; pp 154-155. Thich, J. A.: Toby, B. H.; Powers, D. A,; Potenza, J. A.; Schugar, H. J. Inorg. Chem. 1981, 20, 3314. Giacovazzo, G.: Scordari, F.; Menchetti, S. Acta Cystallogr. 1975, B31, 2171. Cannon, R. D.; White, R. P. Progr. Inorg. Cheni. 1988, 36. 195. Gorun, S . M; Lippard, S. J. J. Am. Chem. Soc. 1985, 107, 4568. Gorun, S. M.: Papaefthymiou, G. C.; Frankel, R. B.: Lippard, S. J. J. Am. Chem. Soc. 1987, 109. 4244. Kitajima. N.; Amagai, H.; Tamura, N.: Ito, M.; Moro-oka, Y.; Heenvegh, K. Penicaud. A,; Mathur. R.; Reed. C. A,; Boyd, P. D. W. Inorg. Chem. 1993, 32, 3583. Berg, J. M.; Holm, R. H. In Iron Sulfur Proreins; Spiro, T. G., Ed; John Wiley and Sons: New York, 1982; Vol. 4. Holm, R. H.; Ciurli, S.; Weigel, J. A. Prog. Inorg. Chem. 1990, 38, 1.

Inorganic Chemistry, Vol. 34, No. 18, 1995 4665 nitrile solutions, 7 does not undergo any reversible redox process in the range -1.5/+1.5 V us Fc+/Fc. Trinuclear complexes of the early-transition elements with sulfide or oxide bridges and a similar overall structure are also

Structural Relevance to Iron Oxides and Hydroxides. Several polyiron(I1,III) clusters exhibiting a closest-packing ( c P )of~ ~oxygen atoms have been isolated in recent years. Two such layers of oxygen atoms from hydroxide, oxide, alkoxide, or phenoxide ligands are found in the cores of the complexes 1, 2, and 9 and of the heptairon(I1) cluster Fe1$(0CH3)6( O P ~ ) ~ ( C H ~ C N(16),34 ) I ~ ~all + of which display the CdI2-type structure.33 Similar cp layered structures have been recently observed in the cores of two nickel(I1) clusters with siloxanolate ligands.35 In complex 7, the 13 oxygen atoms in the first coordination sphere of iron(II1) ions are arranged in two essentially parallel layers. Atoms 0(1), 0 ( 5 ) , 0(6), 0(7), 0(8), 0(9), and O(10) show average deviations of 0.14 8, from the best plane through them (plane a). With respect to the idealized geometry of a hexagonal net, plane a displays significant distortions, which arise from the constraints imposed by the chelating dbm ligands (Chart la). In fact, the shortest distances between oxygen atoms from the same dbm ligand and from direrent dbm ligands average 2.738(3) and 3.37(9) A, respectively, whereas O(i)0 ( 1 ) separations (i = 5, 6, ..., 10) average 3.05(9) A. Less evident distortions affect the mean plane through 0(2), 0(3), 0(4), 0(11), O(12) and O(13) (plane b, see Chart Ib), in which 0 -0 separations average 2.87(3) A. However, larger deviations of the atoms from the best plane are present (0.23 A, after averaging). Planes a and b and the mean plane through the iron atoms (plane c) are parallel within 0.5". Planes a and b are also almost equidistant from the plane through the metals (1.20 and 1.08 A, respectively). A third incomplete layer of oxygen atoms can be envisaged in complex 7. It comprises O( 14), O( 15) and Os (plane d), with 0.* 0 distances larger than 4 A, and is found to be parallel to planes a-c within 1.1". The potassium ion lies very close to plane d (0.50 A) whereas its distance from plane b is much larger (2.20 A). In Chart IC the three layers of oxygen atoms described by planes a, b, and d are depicted. O(16) is also indicated, but it must be remembered that O( 16) lies quite far from plane d. It is seen that the large ionic radius of potassium and hydrogen bond interactions (marked by thin dotted lines) result in dramatic distortions of the arrangement of oxygen atoms of plane d with respect to the idealized positions of a cubic-closest-packing ( c c P ) ,which ~ ~ are indicated by cross hairs. A distorted ccp of oxygen atoms has indeed been observed in complexes 3-6 and in the hexanuclear p6-0XO-Centered cluster^.^^-^^ It is noteworthy that the structure of these molecular aggregates has so much in common with the three-dimensional lattices of metal oxides displaying a ccp of oxygen atoms, such as NiO, FeO and y-Fe203. The related hexagonal-closest-packing (hcp) is found in a-FezO3. a-FeO(OH) (gothite) and y-FeO(0H) (lepidocrocite) contain cp layers of oxygen atoms as The O * * O separations within (32) Muller. A.; Jostes, R.; Cotton. F. A. Angew. Chem., Inr. Ed. Engl. 1980, 19, 875 and references therein. (33) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, England, 1984; p 258. (34) Nair, V. S.; Kitaygorodskiy, A.; Hagen, K. S. Presented at the 206th National Meeting of the American Chemical Society, Chicago, IL, 1993. (35) Levitsky, M. M.; Shchegolikhina, 0.;Zhdanov, A. A,: Igonin, V. A,; Ovchinnikov. Yu. E.; Shklover, V. E.; Stmchkov, Yu. T. J. Organomef. Chem. 1991, 401, 199. Igonin,V. A.; Lindeman, S. V.: Potekhin, K. A.: Shklover, V. E.; Struchkov, Yu. T.: Shchegolikhina, 0.;Zhdanov, A. A.: Razumovskaya, I. V. Organomer. Chem. USSR 1991, 4 , 383.

Caneschi et al.

4666 Inorganic Chemistry, Vol. 34, No. 18, I995 Chart 1 .20 0m. . . . . . . . . Q . . . . . . . . .

o n F -1

.I5

z

2. . I O x

.05 Q

............. 0

. .

*

. . .

0 0

100

200

300

T (K) b

Figure 3. Temperature dependence of x and xT for compound 7 in an applied field of 1 T. The calculated curves for g = 2.0, J = J’ = 8.7 cm-l (dashed line) and for g = 2.0, J = 10.6 cm-’, J’ = 15.3 cm-’ (solid line) are also shown.

+

+

Q

.............

where S23 = S2 S3 and ST= S1 S23. No satisfactory fitting of the experimental susceptibility could be obtained setting J’ = J (i.e. assuming C3v point-group symmetry). In fact, the J value which gives an acceptable fitting to xT,J = 8.7( 1) cm-l, requires a maximum in at about 20 K which is not experimentally observed (Figure 3). Relaxation of the condition J’ = J was then allowed. This has an important physical consequence when J’, J > 0: it partially removes the 4-fold degeneracy of the spin ground state of the system in zero field. In fact, when J’ = J, the energy of the spin states does not depend on the intermediate spin quantum number S23 [see (2)] and for J > 0 the ground state is given by two degenerate spin doublets, arising from S23 = 2 and S23 = 3. However, when J’ J, the energy of the spin states does depend on S23 and the energy gap between the two doublets is given by

x

planes a and b in complex 7 are to be compared with the values found in FeO (3.04 A), in a-Fe203 (2.70-3.04 A) and in a-FeO(0H) (2.74-2.84 A). Magnetic Behavior. The measured molar magnetic susceptibility of 7 in a field of 1 T is plotted in Figure 3 as a function of temperature. The insert shows the temperature dependence of the XTproduct. The susceptibility (0.0345 emumol-I at 270 K) increases steadily with decreasing temperature, reaching 0.2100 emumol-I at 2.2 K. The room-temperature XTproduct (8.93 emu*Kmol-I)is appreciably lower than expected for three uncoupled S = 5/2spins (13.13 emu*Kmol-I). When the sample is cooled down, XT decreases continuously and does not extrapolate to zero as temperature approaches 0 K, as expected for three S = 5/2 spins k T = 0.46 emu*Kmol-I at 2.2 K). The system obeys the Curie-Weiss law down to about 60 K, with C = 13.48 emwKmol-’ and 0 = -137.5 K. A CzVpoint-group symmetry was assumed for the triiron complex and a spin-only Heisenberg-Dirac-VanVleck (HDW) Hamiltonian formalism was adopted to reproduce the observed magnetic behavior.

The usual Kambe36vector-coupling approach gives the energies of the spin states as functions of the exchange integrals J and J’.

(36) Kambe, K. J. Phys. SOC.Jpn. 1950, 5,48.

*

!E(

3) - E( ‘I2,2) I = 3 IJ’ - JI

(3)

A net improvement in the quality of the fit was achieved (Figure 3). However, two distinct minima are present on the relativeerror surface, corresponding to the two sets J = 10.6(1), J’ = 15.3(2) cm-’ (a)and J = 12.9(2), J’ = 9.7( 1) cm-’ (b). In the resulting picture of the spin states, the energy gap between the two lowest-lying doublets is 14.1 and 9.6 cm-I for sets a and b, respectively. As a consequence of the reorganization of the spin states with respect to the C3vapproximation,the first excited multiplet lies 6.5 cm-I above the ground spin doublet and is characterized by S23 = 1, ST = 3/2 for set a and s23 = 4, ST= 3/2 for set b. The estimated magnitude of the exchange integrals, J and J’, compares well with a number of observations on alkoxo-bridged polyiron(II1) complexes. Current treatments of magneticexchange interactions between iron(1II) centers involving oxygen bridging atoms emphasize the role of the coupling distance P, defined as half of the shortest super-exchange pathway between two iron(II1) ions, in determining the antiferromagnetic contributions to magnetic coupling. In multiply-bridged diiron(II1) units, an inverse exponential dependence of J upon P has been established e m p i r i ~ a l l y . ~ Possible ~ super-exchange pathways in compound 7 involve the O(l), 0(2), 0(3), and O(4) oxygen atoms from bridging methoxide ligands. In structure I, we (37) Gorun, S. M.; Lippard, S. J. Znorg. Chem. 1991,30, 1625. Turowski, P. N.; Armstrong, W.H.;Roth, M. E.; Lippard, S. J. J. Am. Chem. SOC. 1990, 112, 68 1.

Polyiron(II1)-Alkoxo Clusters report a sketch of the Fe3(0)4 skeleton from which the length of the various superexchange pathways can be calculated. It is easily seen that for each iron-iron interaction the shortest pathway invariably involves oxygens from pz-OCH3 ligands. Use of the average iron-iron coupling distance (2.005 A) in conjunction with the equation reported in ref 37 leads to Jav= 16.6 cm-I, which is of the right order of magnitude. Unfortunately, it is not possible to draw a conclusion as to which set of coupling constants is more likely to be the right one. Judging from the metrical parameters of the Fe3(0)4 skeleton, the Fe( 1)Fe(2) magnetic interaction through O(2) should be weaker than the other two. The large structural differences between “basic” carboxylates and complex 7 have one major consequence: for iron(II1) “basic” carboxylates typical J values resulting from the equilateraltriangle model (60 ~ m - ’ are ) ~ almost ~ 1 order of magnitude larger than for complex 7 (8.7 cm-I). In fact, though the average iron-iron separation is smaller in complex 7 than in “basic carboxylates” (3.24 us 3.32 A), in the latter the ironiron coupling distance, which involves the central p3-oxide ligand, is shorter than in complex 7 (1.92 vs 2.00 A). As a result, while the highest spin states of iron(II1) “basic” carboxylates are not significantly populated at room-temperature, the energy of the S = 15/2 manifold of cluster 7 is comparable to the thermal energy at room-temperature. Moreover, according to the equilateral-triangle model the first excited multiplet (ST= 3/2) in 7 lies approximately 13 cm-’ above the ground state, whereas in “basic” carboxylates the energy gap can be as large as 50 cm-I and the ground state is therefore much more “isolated’. The results obtained by the application of HDVV Hamiltonians to the interpretation of the magnetic data of compound 7 suggest that, with respect to the predictions of C3L HDVV Hamiltonian, a complete reorganization of the spin states is necessary in order to reproduce the observed magnetic behavior. Within the HDVV formalism this can be successfully achieved by taking into account distortions from the idealized trigonal symmetry. The same situation is commonly encountered in triiron(II1) p3-oxo-centered “basic” carboxylates, in which case it does not seem to arise merely from structural inequivalency of the three exchange pathways. An alternative explanation for the magnetic behavior of 7 retaining an idealized trigonal symmetry is the presence of nonHeisenberg exchange contributions. In triangular arrays of antiferromagnetically-coupled half-integer spins with trigonal symmetry the two S = states, which can be referred to as 2E, are split by spin-orbit interaction. In the spin Hamiltonian approach this corresponds to the introduction of antisymmetric exchange (AS) term^.^^.^^ More generally, AS- and Heisenbergexchange contributions sum up in determining the splitting of the ?E state in a distorted system. A convenient Hamiltonian for a Cll-symmetry array of three interacting spins in a magnetic field H, assuming an isotropic g factor and including ASexchange interactions, has the form

where G is the vector of antisymmetric exchange38and the other symbols have their usual meaning. In the following we will set G, = G, = 0, which may be expected to hold if the (38) Rakitin, Yu. V.: Yablokov, Yu. V.; Zelentsov. V. V. J. Magn. Reson. 1981, 43, 288. (39) Tsukerblat. B. S.; Belinskii. M. I.; Fainzil’berg, V. E. Sou. Sci. Rev. E, Chem. 1987. 9, 337 and references therein.

Inorganic Chemistry, Vol. 34, No. 18, 1995 4667

2.5

3 ..........................................................

31 .............

..............

-2.5

0

2

4

6

H (T)

Figure 4. Field dependence of the eigenvalues of Hamiltonian (4), with the assumptions explained in the text and B/gpB = D/gpe = 2 T (g = 2.0). The arrows show the transitions which are believed to be responsible of the observed EPR signals in compound 7 and 17. The resonance fields are not to scale.

distortions from C3, symmetry are small, and we will assume that the leading contribution to H comes from the Heisenberg exchange terms. For S = 5 / 2 spins the energy of the four magnetic sublevels of the ground state, as resulting from a firstorder perturbation treatment,39is given by

where H = /HI, D = D,= 9(3)1’2G,,6 = 3(J’ - J), A2 = d2 D2 and 0 is the angle between the field and the z axis, which is normal to the plane of the luster.^^.^^ The parameter A thus measures the splitting of the magnetic sublevels in zero field (Figure 4). We tried to apply the above formulas to the ground state of complex 7, while retaining the condition J = J’. However, no net improvement of the quality of the fit resulted. This confirms that the overall picture of the spin levels arising from the C3L, HDVV model is incorrect and that the discrepancies do not merely depend on the splitting of the ground state. X-Band EPR spectra recorded on powdered samples provide useful information about the relative importance of AS- and Heisenberg-exchange contributions to the splitting of the ’E state. At room temperature, a single broad line is observed at g M 2. On cooling down the sample, the intensity of the signal increases gradually and at liquid helium temperature an anisotropic line is present at getf = 1.89 (AH= 575 G) (Figure 5). Since the HDVV model predicts that an isotropic EPR line should be observed at the single-ion g value [g = 2.00 for iron(III)], irrespective of the geometry of the cluster,38 nonHeisenberg contributions must be present. Comparison with the X-band EPR spectra of [Fe30(CH3(17)3xmay be instructive. Two overC0&,(H?0)3]C1*4H~0 lapping lines at gett % 2 and 1.6-1.4 are observed on polycrystalline samples at liquid-helium temperature. They are believed to arise from “parallel” and “perpendicular” transitions involving the four magnetic sublevel of 2E parentage (see Figure 4). The resonance fields calculated by (5) are

+

4668 Inorganic Chemistry, Vol. 34, No. 18, 1995

1

1

1

2000

1

1

3000

1

1

1

1

1

1

4000

1

1

1

1

1

5000

1

Caneschi et al.

1

1

6000

H (G)

Figure 5. Powdered sample X-band EPR spectrum of compound 7 in the range 2000-6000 G. The intensity is arbitrary.

It follows that, as far as 161 > hv, H l increases with increasing ID/ whereas HI[should depend on the single-ion g factor only. Simulation of the EPR spectra of compound 17 showed that ID1 x 161, both parameters being much larger than the microwave quantum energy hv.39 For compound 7 a single, though anisotropic, line is observed at a g,ft. value not far from that expected for the single-ion, which means that ID1 < 161. The shape of the observed signal can be reproduced assuming the linewidth to be markedly anisotropic, with AH1 > AHll. According to this simplified model, Heisenberg-exchange interactions provide the main contribution to the splitting of the ground state of the trimer, therefore supporting the perturbative approach which leads to (5). Significant departures from C3L8 idealized symmetry are indeed observed in complex 7 and it is well-known that, in the limit of weak interactions, the value of J is very sensitive to small angular changes or distortion^.^'

Conclusions The triiron(II1) cluster KFe3(0CH&(dbm)y4CH3OH (7) has been synthesized by reaction of iron(II1) chloride and Hdbm in

the presence of excess potassium methoxide. The arrangement of the oxygen atoms in the core of 7 reproduces a fragment of a cp lattice whose octahedral holes are occupied by the metal atoms. The structural similarities with bulk iron oxides and hydroxides such as a-FelOs, a-FeO(OH), and y-FeO(0H) provide a clear example of the link between nanostructured molecular materials and extended systems. The magnetic behavior of 7 is consistent with the presence of antiferromagnetic exchange interactions between the metal centers. With respect to “basic” iron carboxylates, a much weaker metal-to-metal coupling is observed. This can be rationalized on the basis of the structural differences between 7 and p3-oxo-centered carboxylates. The splitting of the 4-fold ?E electronic ground state of the cluster is attributed mainly to geometrical distortions from trigonal symmetry, which result in significant inequality of the exchange coupling interactions. Antisymmetric exchange contributions, detected by low-temperature EPR spectra, seem to play a minor role, a situation which contrasts with that encountered in [Fe30(CH3C02)h(H20)3]Cl*4Hz0. Our current research is aimed at tuning the experimental conditions so as to favor the growth of these iron(II1)-alkoxide aggregates.

Acknowledgment. We gratefully acknowledge Prof. S. J. Lippard of MIT for a helpful discussion. This work was supported by the Human Capital and Mobility Program through Contract ERBCHRX-CT920080 and by “Minister0 dell’ Universith e della Ricerca Scientifica e Tecnologica” (MURST) of Italy. We are grateful to the “Centro Interdipartimentale di Calcolo Automatic0 ed Informatica Applicata” (CICAIA) of Modena University for computer facilities. Supporting Information Available: A full listing of crystal and experimental parameters and of bond distances and angles, anisotropic thermal parameters for non-hydrogen atoms, fractional coordinates and isotropic thermal parameters for hydrogen atoms, and selected 0. * -0 separations within oxygen layers (10 pages). Ordering information is given on any current masthead page.

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